1. Field of the Invention
The present invention relates to an improvement on a technique for measuring the weight of a crystal in a cable-type crystal pulling apparatus wherein a load cell is incorporated into a cable winding mechanism so as to measure the weight of the crystal.
2. Description of the Related Art
In the field of cable-type crystal pulling apparatuses, there has conventionally been used a technique for measuring the weight of a crystal through use of a load cell incorporated into a cable winding mechanism. This technique makes it possible to control the diameter of a crystal based on variations in the weight of the crystal and to measure the weight of the crystal after completion of pulling operation. The accuracy of such a load cell is normally represented by an error relative to a full scale deflection (i.e., the maximum output of the load cell). Consequently, for example, when a load cell having a greater full scale deflection is used to weigh a heavier crystal, accuracy of the load cell is degraded. By contrast, when a load cell having a large capacity or a wide measuring range and a high accuracy is used, the cost of the load cell increases.
Examples of a technique for incorporating a load cell into a cable winding mechanism are disclosed in Japanese Patent Publication (kokoku) No. 5-19087 and in Japanese Patent Application Laid-Open (kokai) No. 7-172980.
According to the former technique, as shown in FIG. 3, a cable winding mechanism has a structure such that a cable 51 for pulling a crystal is wound onto a winding drum 53 via a guide pulley 52, which is subjected to a load (2 W) twice as large as the weight (W) of the crystal. The guide pulley 52 is supported by a load plate 54, which, in turn, is supported by a plurality of supports 55, 56, etc., so that the applied load is equally distributed to the supports 55, 56, etc. The thus-distributed load is detected by a load cell 57 which is disposed on the selected support 55.
According to the latter technique, as shown in FIG. 4, a guide pulley 52 is rotatably attached to the free-end of a horizontal arm 59, other end of which is pivotably supported by a support 58. The free-end of the arm 59 is suspended from above via a load cell 57. After the running direction of a cable 51 is changed from vertical to horizontal by the guide pulley 52, the cable 51 is wound onto a winding drum 53. In this structure, a load cell whose full scale deflection corresponds to the weight (W) of a grown crystal can be used as the load cell 57.
However, in the former technique, it is assumed that a load is equally distributed to the individual supports 55, 56, etc. Thus, this technique is likely to produce an error in measurements.
In the latter technique, although a load cell whose full scale deflection corresponds to the weight of a grown crystal can be used, a load cell having a large capacity and high accuracy is required as the weight of a crystal increases. In addition, it has disadvantages such as a complex mechanism due to employment of a link mechanism, difficulty in maintaining the initial accuracy at the time of assembly, and relatively difficult maintenance work.
With the advancement of semiconductor technology in recent years, demand for pulling a crystal having a larger diameter and a heavier weight has been growing significantly. Under the circumstances, there has been eager demand for a technique for measuring the weight of a crystal through use of a simple and low-cost structure having a sufficient accuracy.